The use of powder metallurgical techniques has become popular with high alloyed materials due to the problems encountered in casting such materials, e.g., segregation and resulting loss of physical properties. For example, powder metallurgical techniques are used extensively with nickel, cobalt, and ferrous-base superalloys. These are high temperature -- high strength alloys used for making turbine discs, blades, buckets, and other components of jet engines which are subjected to high stress at mid-range or high temperatures. The very properties which make these alloys attractive for use in jet engines cause the consolidation of the powders to be difficult. Moreover, subsequent operations, such as forging and machining the resulting densified compact, to produce a final part are also difficult because of the high strength and toughness of these alloys.
Due primarily to the difficulties encountered in post-consolidation processing, efforts have been made to produce "near net shapes". As used herein, a near net shape is a densified powder metal compact having a size and shape which is relatively close to the desired size and shape of the final part. Heretofore, crude preforms have been produced which require extensive forming and machining to produce the relatively complex final part. Producing a near net shape reduces the amount of post-consolidation processing required to achieve the final part. For example, in many instances subsequent hot forging may be eliminated and the amount of machining required may be significantly reduced. Since these materials are difficult to machine, a reduction in the amount of machining offers a marked savings in tool and labor costs. Additionally, these materials are quite expensive, therefore, a reduction in machining results in a savings in material costs. Obviously, eliminating or reducing the amount of hot forging also offers savings advantages.
While the desirability of producing near net shapes has been recognized, many problems have been encountered in accomplishing this objective. The basic step of consolidating the metal powder to produce a powder metal compact having a near net shape has been a major obstacle. Once an acceptable near net shape is produced, other problems are presented. One of these relates to the heat treatment of the densified compact to achieve maximum physical properties.
Due to the fact that a near net shape is being produced, the configuration of the densified compact is relatively complex. Hence, the section size of the densified compact may vary greatly. As is well-known in the heat treating art, variations in section size may cause distortion and internal stresses in the densified compact due to differences in the rates of heating and cooling. The rate of heating also affects time at temperature which is determintive of the physical properties of the heat treated compact. Thinner sections, which reach temperature first, will be subjected to a longer holding period at temperture than thicker sections. This may result in significant, and most likely undesirable, differences in physical properties in various sections of the compact. For example, in an alloy strengthened by age hardening, overaging may occur in the thinner sections. Relative cooling rates are also critical in achieving a relatively uniform microstructure. Additionally, where heat treat temperatures approach the fusion temperature of the lowest melting constituent, the densified compact will become subject to deformation under relatively low stresses. Therefore, the densified compact is easily distorted. This problem is particularly acute in thinner sections which may deform under their own weight. Other problems associated with heat treating parts of complex shape should be immediately apparent to those knowledgeable in the art.